This invention relates to a semiconductor integrated circuit device and, more particularly, to a substrate potential control circuit for use in the semiconductor integrated circuit device.
With high integration in a semiconductor integrated circuit device, it is necessary for a sense amplifier used therein to detect a minute potential difference between digit lines. More specifically, in order to make the integration high, a cell of one-transistor and one-capacitor type is used as a memory cell for use in a current typical dynamic random access memory (which is abbreviated to DRAM hereinunder). As is well known in the art, the cell of one-transistor and one-capacitor type comprises two elements, a capacitor element for accumulating electric charges and a metal oxide semiconductor field effect transistor (MOSFET) for controlling input/output of the electric charges. With high integration, the capacitor element necessarily has a small capacitance value and, consequently can only charge a small amount. Accordingly, a sense amplifier, which is for detecting the presence of absence of the electric charges accumulated in the capacitor element, must detect a potential difference (which is also called a difference potential) defined by the trace of electric charges which are accumulated in the capacitor element. For instance, in the DRAM having storage capacity of four Mbits, the above-mentioned potential difference is about 200 millivolts (or, a very minute amount).
As a result, it is difficult to detect the difference potential in operation, part due to soft error, variation of power-supply voltage, or the like, which could result in data being destroyed. Accordingly, a recent development is to employ a sense amplifier with a dummy word. The sense amplifier of this type operates by making the difference potential large by coming down due to capacitive coupling a level of one of the digit lines that acts as a basis on sense operation.
As described below, it is necessary for the memory cell to control a threshold voltage of the MOSFET. To control the threshold voltage, a substrate potential may be controlled, because the threshold voltage is defined by the substrate potential. Known substrate potential control circuits are for controlling the substrate potential to make the threshold voltage constant. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 4-38,791 or JPA 4-38,791 discloses a semiconductor device which is capable of maintaining the substrate potential at a set voltage in spite of variation of an external power-supply voltage. That is, the JPA 4-38,791 makes an internal voltage having less dependence on the external power-supply voltage, and produces, on the basis of the internal voltage and an actual substrate potential, a substrate potential detection signal. As a result, it is impossible to the JPA 4-38,791 to change the substrate potential in response to the power-supply voltage. This is because the substrate potential is maintained constant although the power-supply voltage varies.
A conventional substrate potential control circuit, which can make the substrate potential change in response to the power-supply voltage, is known. The substrate potential control circuit comprises a substrate potential detection circuit, a back bias generation circuit, and a pumping circuit. The substrate potential detection circuit detects the substrate potential to produce a substrate potential detection signal. Responsive to the substrate potential detection signal, the back bias generation circuit generates a back bias signal. Responsive to the back bias signal, the pumping circuit carries out a pumping operation to make an absolute value of the substrate potential larger. The combination of the back bias generation circuit and the pumping circuit serves as a substrate potential generation circuit for generating the substrate potential in response to the substrate potential detection signal.
However, the conventional substrate potential control circuit cannot control so as to make the absolute value of the substrate potential smaller when the power-supply voltage has a minimum level, and to maintain the substrate potential when the power-supply voltage has a maximum level. Primarily because the connection between the power-supply voltage and a substrate potential detection level is approximately linear in the conventional substrate potential control circuit.